Many architects prefer to use flush expansion joint covers wherever possible for good appearance and a minimum of discontinuity in the exposed surface of the joint. A preferred type of flush expansion joint cover is one that employs a compressible, extendable gasket between one or both edges of the cover member and the corresponding frame member. The gaskets maintain substantial continuity of the elements of the cover upon expansions and contractions of the joint and also provide an air and liquid seal for isolating the building interior from the expansion space. In such expansion joint covers, the surface of the cover member lies flush with the plane of the surfaces of the building members on either side of the joint.
The maximum excursion toward and away from each other of the building members at the expansion gap for a cover with one gasket is about two inches (one inch compression and one inch extension), which is rarely exceeded in conventional building designs. Expansion joint covers with gaskets between both edges of the cover member and the respective frame members can be used for excursions of up to four inches. In buildings designed to withstand earthquakes (seismic events), however, the expansion joints are virtually always designed to endure excursions of the building members at the joint of more than four inches and may be designed for excursions of up to 20 inches. Although conventional flush expansion joint covers of the type with gaskets and flush cover members can be used in seismic expansion joints and will serve entirely satisfactorily under normal excursions of the building members at the joint due to thermal expansions and contractions of the structure and movements due to wind loads, a significant seismic event will almost certainly severely damage the expansion joints, due to dislodgement coupled with rupture of the gaskets and to buckling of the cover members caused by forced contacts with the frames of the joint covers upon closures of the expansion gap.